Time zones divide the world into regions that observe the same standard time, enabling coordination across the globe. Understanding how they work is essential for global applications.
How Time Zones Work
The Earth rotates 360 degrees in 24 hours, or 15 degrees per hour. Time zones roughly follow these 15-degree longitude bands, offset from UTC (Coordinated Universal Time). Common offsets include UTC+0 for London (winter), UTC-5 for New York (EST), UTC-8 for Los Angeles (PST), UTC+9 for Tokyo, and some unusual ones like UTC+5:30 for India.
IANA Time Zone Database
The authoritative source for time zone data is maintained by ICANN and updated multiple times per year. It uses "Continent/City" format (e.g., America/New_York) because country boundaries change, time zone rules change, but cities rarely change names. All major operating systems and software use this database.
Daylight Saving Time
About 70 countries adjust clocks seasonally, springing forward (losing 1 hour) and falling back (gaining 1 hour). DST dates vary by country and change year to year. The southern hemisphere reverses timing. Some regions within DST countries have opted out (like Arizona in the US).
Notable Time Zone Facts
China uses a single time zone despite spanning 5 geographic zones. France has 12 time zones due to overseas territories. Kiribati (UTC+14) is first to see midnight; Baker Island (UTC-12) is last. Some zones use unusual offsets: India UTC+5:30, Nepal UTC+5:45, Iran UTC+3:30, Newfoundland UTC-3:30.